Three-axis stabilized spacecraft are widely used for various communication and sensing systems. Attitude control of the spacecraft may be accomplished by accelerating or decelerating momentum wheels or reaction wheels, by chemical thrusters, or by magnetic torquers, or by combinations thereof. In addition to accelerating or decelerating a wheel to generate a torque about its spin axis, the wheel may be mounted to the spacecraft by a pivot, whereupon a body torque may be generated at right angles to the spin axis by rotating the wheel mounting relative to the spacecraft body.
A prior art spacecraft 10 in FIG. 1 includes roll, pitch and yaw axes 20, 16 and 18, respectively, and earth sensor assembly 22 directed toward Earth 12 for determining pitch and roll, and a momentum wheel assembly (MWA) 24 mounted with its spin axis 26 approximately orthogonal to the orbital plane defined by axes 18 and 20, with the momentum wheel mounting 28 being pivotable about the roll axis 20. In such a system, the roll-representative signal from the earth sensor assembly (ESA) is processed by a control law, such as a proportional-integral (PI) control law, and is applied to drive a motor 30 affixed to the momentum wheel pivot, so as to adjust the roll position of the spacecraft in a feedback manner. As a result of external forces acting on the spacecraft, due to solar pressure, magnetic fields and the like, the pivot angle may increase in a secular manner, which is an increasing amplitude oscillation at the orbit rate. This may result in a loss of control authority when the mechanical limits (not illustrated) of the pivot system are reached.
To prevent loss of control authority, prior art systems, such as are described in U.S. Pat. No. 4,916,622, which is herein incorporated by reference, provide for automatic unloading of the pivot wheel angle. This is accomplished, in short, by sensing the pivot angle, and by, when the pivot angle exceeds a particular value, energizing a torquer in a direction which reduces the pivot wheel angle. Such prior art pivot angle unloading systems sense the pivot angle and feed a signal representative of the pivot angle to a torquer for driving the torquer in a direction which returns the pivot angle towards zero. In FIG. 1, the torquers are represented by magnetic coils 32, 34 and 36.
An improved attitude control system is desired.